The proposed research will be a physical/chemical investigation of protein-protein interactions involved in regulation of contractile activity in skeletal, cardiac, smooth, and non-muscle cells. A survey of the shape of regulatory and essential myosin light chain subunits and the effects of phosphorylation on the interaction between the regulatory light chain and the heavy chain of myosin and on the conformation of both the isolated light chain and the whole myosin and its regulated proteolytic subfragments using sedimentation velocity and equilibrium ultracentrifugation, analytical gel permeation chromatography, fluorescence anisotropy decay measurements, circular dichroism, nuclear magnetic resonance and electron spin resonance spectroscopy. This work will help answer the question of how the regulatory subunits interact with each other after being activated by calcium ion to allow actin-myosin interaction, ATP hydrolysis and, ultimately, contraction to occur.